This invention relates to windmills, that is to say apparatus that include a wind driven rotor and are used to convert the kinetic energy of the wind into useful work or other forms of energy.
More particularly, the invention relates to such apparatus wherein the rotor is incorporated in a kite, that is to say a tethered flying platform that may be supported entirely by its interaction with a wind of appropriate strength. Apparatus of that kind is referred to as a windmill kite herein.
A windmill kite comprising a high flying platform incorporating two wind driven rotors, two dynamos mounted on the platform respectively driven by the rotors, three tensile tethering lines extending from the platform to ground level anchorages and conductive cables connecting the dynamos to a ground-based electricity transmission system or other electric load, has been previously proposed by the present inventor and others.
The term xe2x80x9cdynamoxe2x80x9d is used in the preceding paragraph and hereinafter in its generic sense as a machine for converting mechanical energy into electrical energy, or vice versa. The term designates a reversible machine; any dynamo may be used either as a generator or as a motor. Moreover, the term covers both AC and DC machines.
The purpose of incorporating the wind driven rotors in a kite is to enable them to be positioned at a high altitude where relatively strong and continuous winds may be expected to be found.
Each wind driven rotor (referred to simply as xe2x80x9ca mill rotorxe2x80x9d hereinafter) comprises a rotatable hub and a plurality of equi-angularly spaced blades extending radially from the hub. Preferably each blade is of aerofoil section and blade pitch control means are provided whereby the angle of attack of the blades may be adjusted from time to time.
Mill rotors are also reversible machines. On the one hand, wind directed through the swept area of the blades induces a continuous rotor torque, enabling the mill rotor to, for example, drive an associated dynamo as a generator. On the other hand, rotation of the mill rotor in still air by a dynamo acting as a motor induces a continuous air flow through the swept area producing a thrust force, enabling the mill rotor to, for example, lift itself and the dynamo from the ground.
That previously proposed windmill kite was described and evaluated in a paper entitled xe2x80x9cFlying Electric Generator to Harness Jetstream Energyxe2x80x9d delivered by the present inventor to the xe2x80x9cSpace 2000xe2x80x9d Conference of the American Society of Civil Engineers held at Albuquerque USA in February-March 2000, and is presently available in the published proceedings of that Society at pp 1020-6.
The windmill kite described in that paper comprised an elongate flying platform including two outrigger mill rotors disposed symmetrically on each side of the longitudinal centre line of the platform. Each mill rotor was connected by a geared drive-transmission connecting it to a dynamo carried by the platform.
The flying platform, as illustrated in that paper, has a leading end and a trailing end relative to the direction of the wind, with adjustable but essentially fixed, preferably aerofoil, control surfaces at the ends of the platform. Those control surfaces are intended to ensure that the leading end of the platform is directed into the wind and that a transverse axis extending through the centres of the mill rotors remains substantially horizontal. Those control surfaces are also intended to ensure that the relative elevations of the ends of the platform are maintained so as to produce a substantially constant pitch angle for the platform as a whole, at which the wind flows obliquely against the undersides of the mill rotors.
As described in that paper, under those circumstances a drag force acts upon the mill rotors and platform, and a thrust force having an upward component and a torque are developed by each mill rotor. The upward component of the thrust forces and drag forces are opposed by the weight of the platform and the items carried by it and by tension in the tethering lines extending from the platform to ground level anchorages.
Those anchorages include a winch or winches that enable the flying height of the platform to be adjusted as needed.
The torque reactions from the mill rotors acting on the platform are cancelled out by ensuring that the mill rotors rotate in opposite directions. The generated torque is used to drive the dynamo or dynamos when operating in the generator mode.
A major problem with that prior proposal arises from the fact that, even at high altitudes, winds sometimes fail to blow with sufficient strength to enable those control surfaces to adequately stabilise the flying platform against variables such as wind gusts or eddies. Thus, should the wind fail, it has been necessary for the platform to be winched down to prevent the tethering lines and/or conductive cables from becoming tangled, and, in a worst case scenario, to prevent the platform from crashing. Winching the platform down and subsequently returning it to an operating altitude is a time consuming and expensive operation
An object of the invention is to provide a windmill kite for generating electrical power from wind energy that overcomes the previously found problem of platform stability at times of weak or no wind.
Another object is to lessen the need to winch the platform down at times of calm.
The invention achieves the first mentioned object by dispensing with static, albeit adjustable, control surfaces;
providing at least three strategically placed mill rotors;
providing blade pitch control means to independently control the angle of attack of the blades of each of the mill rotors, to thereby stabilise the platform irrespective of whether the dynamos are functioning as generators or as motors for the time being.
The invention achieves the second mentioned object by enabling electric power to be delivered to the dynamos so that they may function as motors and drive the mill rotors for short periods to enable the platform to stay aloft.
Therefore, according to one aspect, the invention consists in a windmill kite of the kind comprising a flying platform including a plurality of mill rotors, at least one tethering line maintaining the platform at a substantially fixed geographical location, at least one dynamo on the platform drive-connected to said mill rotors, and conductor means connecting said at least one dynamo to an electrical transmission and supply system at ground level, characterised in that;
there are at least three substantially axially co-directed, spaced apart mill rotors disposed in an array which is symmetrical in terms of thrust capacity about each of two orthogonal axes, namely an X axis extending longitudinally of the platform, and a Y axis extending transversely of the platform, and is neutral in terms of torque capacity about a third orthogonal axis, namely a Z axis perpendicular to the X and Y axes; and
in that blade pitch control means provide for adjustment of the angles of attack of the blades of the respective mill rotors, to thereby stabilise the platform in a desired attitude and orientation relative to the wind direction.
The symmetry of the array ensures that in normal steady state operating conditions there is no tendency for the platform to roll about the X axis, pitch about the Y axis or rotate about the Z axis. However the individual adjustment of the mill rotors in respect of thrust and torque enables corrective pitch, roll and orientation movements to be induced to cancel adventitious variations.
Preferably the origin of the three axes coincides with the centre of gravity of the platform as a whole.
Theoretically, the requisite symmetry of the array may be achieved by as few as three mill rotors in the array, for example an array wherein a mill rotor that has twice the thrust and torque generating capacity of each of the others coincides with say the X axis and is spaced forwardly of the origin, and the others are equally spaced from the X axis in the Y axis direction and are spaced rearwardly of the origin to the same extent as the first mentioned rotor is spaced forwardly thereof.
However, for the sake of simplicity it is preferred that all of the mill rotors be of equal weight and capacity, in which event there are at least four mill rotors in the array. Preferably the array is a square or rectangular, that is to say each mill rotor is positioned at a respective corner of a notional square or rectangle with its centre point coinciding with the origin of the X, Y and Z axes. In such arrangements it is also preferred that each mill rotor has an associated dynamo and is substantially co-axial therewith.
In large scale embodiments of the invention the ground level load would be a sub-station of a ground-based electricity transmission or distribution system. In that event the windmill kite may operate as a pollution free, base load generator, continuously feeding into the system, with more easily regulated fuel powered or hydro stations contributing to the system at peak load times, and enabling the kite to draw power from the system when and if needed. Under those circumstances a plurality of geographically widely spaced windmill kites may be connected to the one ground transmission system to provide a reliable input for the system as a whole.
According to a second aspect, the invention consists in a method of stabilising the platform of a windmill kite according to the first aspect of the invention by independently adjusting the angle of attack of the blades of the respective mill rotors in the said array.
It is mentioned that the mill rotors of the arrays referred to above are essential for stability control, nevertheless there may be further, possibly non-adjustable, mill rotors in other embodiments of the invention, providing those further rotors are paired and designed to be thrust moment and torque neutral with respect to the relevant X, Y and Z axes.